Phytoremediation of a Nitrogen-Contaminated Desert Soil by Native Shrubs and Microbial Processes

We combined phytoremediation and soil microbial nitrification and denitrification cycles to reduce nitrate and ammonium levels at a former uranium mill site near Monument Valley, Arizona. Ammonia used in uraxnium extraction was present throughout the soil profile. Sulfate, applied as sulfuric acid to solubilize uranium, was also present in the soil. These contaminants were leaching from a denuded area where a tailings pile had been removed and were migrating away from the site in groundwater. We planted the source area with two deep-rooted native shrubs, Atriplex cansescens and Sarcobatus vermiculatus, and irrigated transplants for 11years at 20% the rate of potential evapotranspiration to stimulate growth, then discontinued irrigation for 4years. Over 15years, total nitrogen levels dropped 82%, from 347 to 64mgkg(-1). Analysis of N-15 supported our hypothesis that coupled microbial nitrification and denitrification processes were responsible for the loss of N. Soil sulfate levels changed little; however, evapotranspiration reduced sulfate leaching into the aquifer. For arid sites where traditional pump-and-treat methods are problematic, the Monument Valley data suggest that alternatives that incorporate native plants and rely on vadose zone biogeochemistry and hydrology could be a sustainable remediation for nitrogen contaminated soil. Copyright (c) 2016 John Wiley & Sons, Ltd.